GEOGRAPHIC AND SEGMENT INFORMATION - Reportable Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Sales	$ 6,954	$ 7,575	$ 16,247	$ 19,853
Income (loss) from operations	(4,371)	(4,756)	(18,030)	(14,908)
Interest income	10	327	233	1,153
Interest expense	(25)	0	(39)	0
Other income (loss), net	(63)	230	349	(265)
Income tax (benefit) expense	(715)	171	(7,112)	253
Operating Segments | Advanced Energy
Segment Reporting Information [Line Items]
Sales	5,479	6,094	12,332	15,815
Income (loss) from operations	(1,095)	(1,357)	(8,371)	(5,653)
Interest income	0	0	0	0
Interest expense	0		0
Other income (loss), net	0	0	0	0
Income tax (benefit) expense	0	0	0	0
Operating Segments | OEM
Segment Reporting Information [Line Items]
Sales	1,475	1,481	3,915	4,038
Income (loss) from operations	556	268	1,387	995
Interest income	0	0	0	0
Interest expense	0		0
Other income (loss), net	0	0	0	0
Income tax (benefit) expense	0	0	0	0
Corporate & Other
Segment Reporting Information [Line Items]
Sales	0	0	0	0
Income (loss) from operations	(3,832)	(3,667)	(11,046)	(10,250)
Interest income	10	327	233	1,153
Interest expense	(25)		(39)
Other income (loss), net	(63)	230	349	(265)
Income tax (benefit) expense	$ (715)	$ 171	$ (7,112)	$ 253